The present invention relates to wireless digital communication systems. More particularly, the present invention is directed to a code division multiple access (CDMA) communication system utilizing uplink power control for adaptive modulation and coding.
CDMA third generation (3G) cellular telecommunication systems apply adaptive modulation and coding (AMandC) to transmissions to achieve and improve radio resource utilization and provide increased data rates for user services. AMandC techniques take into account RF propagation conditions in advance of transmissions in order to determine modulation and coding rates that will take greatest advantage of current RF propagation conditions.
One method for determining RF propagation conditions is to perform a physical channel quality measurement at the receiver in advance of each transmission. This measurement is sent to the transmitter, which then determines the appropriate modulation and coding rate for the particular transmission based upon the physical channel quality measurement.
RF propagation conditions can change rapidly, particularly for mobile applications. Since the quality measurement of the radio interface is used to determine the appropriate modulation and coding, and since the channel quality measurement can change rapidly due to the changing RF propagation conditions, the performance of the adaptive transmission process is directly related to the time period (i.e. latency) between when a quality measurement is performed and when that transmission is initiated. Therefore, for optimal AMandC, it is necessary to perform channel quality measurements with minimal latency for all users with active data transmissions.
Physical or logical control channels are used to transfer channel quality measurements from a receiver to a transmitter. Channel quality signaling may utilize either dedicated control channels to each user equipment (UE) or common control channels shared by all UEs. When dedicated control channels are used, a continuous signaling channel is available over time for propagation of channel quality measurements for each UE. In terms of performance, this is an optimal solution for AMandC since the quality measurement is continuously available. Transmissions can occur at any time, taking into account the continuously available quality measurement for appropriate modulation and coding settings. Additionally, with a dedicated control channel always available in the uplink, the channel can be also used to support low rate uplink data transmissions.
The difficulty with the dedicated control channel approach is that physical resources are continuously allocated even when there is no data to transmit. A primary application of AMandC techniques are non-real time high data rate services, for example, Internet access. For these classes of service, the best quality of service (QoS) is achieved with short, high rate transmissions with relatively long idle periods between each transmission. These long idle periods result in an inefficient use of dedicated resources.
The problem can be minimized with pre-configured periodic dedicated channel allocations. But this results in periodic unavailability of quality measurements. If the quality measurements are not continuously available, for UEs which have transmissions at any one point in time, only some portion of the UEs will have recent channel quality measurements.
When common control channels are used, a continuous signaling channel is shared by all UEs within a cell. In Third Generation-Time Division Duplex (3G TDD) systems, the uplink common control channel typically occupies a single time slot out of multiple time slots. Procedures are defined for each UE""s access to the common control channel and UE identities may be used to distinguish UE specific transactions.
To avoid contention-based access to the uplink common control channel, individual allocations are required to be signaled on the downlink common control channel. Alternatively, some mapping between the downlink allocation and uplink allocation may be defined. Each UE then accesses the uplink common control channel in accordance with its allocation. Since uplink transmissions cannot always be predicted by the network, and since uplink transmissions are infrequent, (in some applications transmitting only 5% of the time), periodic allocations of the uplink common control channel are also necessary for propagating uplink radio resource requests to support uplink user data. Additionally, when common control channels are used for AMandC operation, no inner loop power control mechanism exists for each UE, since the common control channels are not continuously available.
What is needed is an efficient method of performing power control while minimizing the overhead necessary to perform such a method. Power control will minimize the interference introduced by the uplink common control channel.
The present invention is a base station which receives a communication signal over an uplink common control channel and measures interference on the uplink common control channel in response to the received signal. A channel quality measuring device included in the base station monitors the channel quality of the uplink common control channel and provides a quality margin for the communication signal. A spreading device, then, transmits the quality margin over a downlink control channel in response to the quality margin.